The present invention relates to a dot-matrix type printer, and more particularly to a printer having at least two sets of printing elements.
A dot-matrix printer is one which forms characters from a plurality of dots arranged in rows and columns. One format known in the prior art utilizes a 7.times.7 matrix, that is, a matrix 7 dots high by 7 dots wide.
A conventional way of printing such characters is to force the ends of wires, or styli, into contact with a printing medium such as an inked ribbon, which in turn makes dot marks on paper. The arrangement of the dots within the matrix conveys the form of the characters. The styli are electromagnetically driven and one popular embodiment includes the styli arranged in columnar form, i.e. a column of 7 styli. The printing line is scanned at constant speed and the styli at appropriate rows in the column are actuated at the appropriate times to form the desired characters.
The maximum printing speed of dot matrix printing systems is related to the maximum allowable repetion rate of dots formed along any given horizontal line. If dots can be formed at the rate of 1000 per second, one character (in a 7.times.7 matrix format) is printed for 7/1000 second.
The printing speed can be doubled by arranging the styli in two columns. Such a system is disclosed in U.S. Pat. No. 4,079,824, for example. Further, the use of plural columns of styli is disclosed in U.S. Pat. Nos. 3,627,096, 4,010,835 and 4,169,683, Japanese Patent Publication No. 57-43428, and Japanese Patent Laid-Open Publication No. 53-145727, for example.
Further, a technique using two columns of styli to quadruple a printing speed as compared with one column of styli is disclosed in U.S. Pat. No. 4,167,342. In this technique, the dots forming a character are assigned to two groups in such a manner that two or more dots within successive four columns are not assigned into a group. The styli in each column are actuated according to dots assigned to a group. Thus, the printing speed is quadrupled as compared with one column of styli. However, this technique requires the preliminarily design of a character generator for each of the two columns of styli per character.